No, it doesn't state outright that having all of them highly rated is not allowed, but that's why I said "suggestion" rather than rule. The last sentence there makes the designer's stance on the issue very clear though. "You could choose to build a spellcaster that's merely medium rated in all three" implies that choosing a spellcaster that is highly rated in all three isn't really a good option as far they are concerned. As does the sentence before that. I doubt they included that box just for the hell of it. In my opinion, they did it because it addresses some of the problems I commonly see come up regarding spellcasters . . . including this one.
That's not making a suggestion or a value judgment at all.
What it's saying is that, if you're building a practitioner, you're not going to be able to have all of them at the top -- a simple statement of fact -- it's impossible to have all three of them at the skillcap in any of the given power levels in the book. You could have all three at 3 or 4, but you can't get all three to 5, because you'd need 45 total points.
And if you're building someone who's well balanced outside of spellcasting, then the room at the top runs out quickly (great, you've got Conviction, Discipline, and Lore at the top three, but now you have to choose the rest of your skills, and they're all starting at 3 and below).
I don't see a need to de-power evocators, personally. If you're having an issue with overpowered casters and don't see the above problem come up very often, then in my opinion, you're running the table wrong for a group w/ a blaster in it. A stacked evocator can do serious, serious things in combat...but they also only get 4 spells off at best before they start needing to take consequences (barring enchanted items but that's another discussion).
Yeah, that's what I was saying -- though in my case, for the most part my wizards end up hurting themselves just by overdoing it on the power (one is, in character, almost suicidally eager to push her limits and tends to blow out an internal gasket when facing something big).